https://techcrunch.com/2018/05/25/googles-duo-and-ciscos-webex-teams-among-the-voip-apps-pulled-from-the-china-app-store/

Earlier this week, it came to light that Apple had removed a number of VoIP-based calling apps from the App Store, at the request of the Chinese government. The apps had been using CallKit, Apple’s new developer toolset that provides the calling interface for VoIP apps, freeing up developers to handle the backend communications. China’s government asked developers, by way of Apple, to remove CallKit from their apps sold on the China App Store, or they can remove their apps entirely.

Notices Apple sent out to the developers were first spotted by 9to5Mac, who shared a snippet from of one of the emails.